This application is related to Japanese application No. 2001-004575 filed on Jan. 12, 2001, Japanese application No. 2001-030028 filed on Feb. 6, 2001, Japanese application No. 2001-394695 filed on Dec. 26, 2001 and Japanese application No. 2001-394695 filed on Dec. 26, 2001, whose priority are claimed under 35 USC xc2xa7119, the disclosure of which are incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a thin-film solar cell and its manufacturing method, and more particularly to a thin-film solar cell having a high photoelectric conversion efficiency and its manufacturing method.
2. Description of the Prior Arts
Attention has been put on a solar cell as alternate energy of fossil fuel such as petroleum that has a problem of having a worry about supply and demand in the future as well as a discharge of carbon dioxide which is a cause of global warming.
This solar cell utilizes a p-n junction at a photoelectric conversion layer that converts optical energy into electric power. Silicon has frequently been used in general as a semiconductor constituting this p-n junction.
It is preferable to use single crystalline silicon from the viewpoint of photoelectric conversion efficiency. However, it has a problem concerning a material supply and production of large areal solar cell at low cost.
On the other hand, a thin-film solar cell having a photoelectric conversion layer has been realized, the photoelectric conversion layer being made of amorphous silicon that is advantageous for realizing a solar cell with a large area at low cost.
Further, an investigation has been made for a use of crystalline silicon to a photoelectric conversion layer so as to realize a solar cell achieving both of a high, stabilized photoelectric conversion efficiency to a level of a single crystalline silicon solar cell and a large area and low cost at a level of an amorphous silicon solar cell. In particular, attention has been made on a thin-film solar cell (hereinafter referred to as crystalline silicon thin-film solar cell) in which a crystalline silicon thin-film is formed by using a thin-film forming technique by a chemical vapor deposition (hereinafter referred to as CVD method) like the case of amorphous silicon.
For example, Japanese Unexamined Patent Application No. HEI1-289173 discloses a multi-junction type thin-film solar cell made by laminating a photoelectric conversion element having an active layer of amorphous silicon and a photoelectric conversion element having an active layer of polycrystalline silicon that has a smaller energy gap compared to amorphous silicon. The thin-film solar cell of this type has a structure that solar light is incident from the side of the photoelectric conversion element having an amorphous silicon active layer, thereby having an advantage of more efficiently utilizing solar light compared to a single-junction type. Moreover, a plurality of photoelectric conversion layers is connected in series, to thereby obtaining a high open-circuit voltage. This can make the amorphous silicon active layer thin, so that a deterioration per hour of the photoelectric conversion efficiency attributed to Staebler-Wronski effect can be reduced. Moreover, this type of solar cell has an advantage of manufacturing the amorphous silicon layer and the crystalline silicon layer by the same device. As described above, the thin-film solar cell of this type has various advantages, and therefore, research and development has popularly been made as a means for realizing a highly efficient thin-film solar cell manufactured also at a low cost.
One of the important techniques for realizing a highly efficient thin-film solar cell includes a light confinement effect. The light confinement effect means that irregularities are made on a surface of a transparent conductive layer or a metal layer that is in contact with the photoelectric conversion layer for scattering light at its boundary, thereby extending optical path length for increasing an amount of light absorption at the photoelectric conversion layer.
For example, Japanese Patent No. 1681183 or No. 2862174 discloses a technique for obtaining a substrate for a highly efficient solar cell by defining a size of grain diameter or a size of irregularities of the transparent conductive layer formed on a glass substrate.
An improvement of the photoelectric conversion efficiency by the light confinement effect serves for decreasing a thickness of the photoelectric conversion layer. This can restrain a photo-deterioration, especially in an amorphous silicon solar cell. Further, this can greatly shorten a film-forming time in the case of a crystalline silicon solar cell that requires, due to its light absorption characteristics, a thickness of no less than several micron orders which is several to ten several times thick compared to amorphous silicon. Specifically, the light confinement effect enables to enhance all of high efficiency, stabilization and production with low cost that are significant problems for realizing a thin-film solar cell.
However, the photoelectric conversion efficiency of the crystalline silicon thin-film solar cell is only the same level as that of the amorphous silicon solar cell in spite of an energetic research and development made so far.
H. Yamamoto et al reported in PVSC-11, Sapporo, Japan, 1999 that forming a microcrystalline silicon layer with a plasma CVD method on an Asahi-U substrate obtained by laminating on a glass substrate tin oxide having minute irregularities thereon stimulates a priority growth of silicon grain in a perpendicular direction with respect to the surface of the tin oxide, which produces collisions between crystal grains each growing from different surfaces of irregularities and having a different crystal orientation, thereby generating a great amount of defects. Such defects remarkably deteriorate the photoelectric conversion efficiency, since they become a recombination center of a carrier.
H. Yamamoto et al simultaneously reported that laminating further zinc oxide thickly on the tin oxide having a surface irregularities for making the depth of each irregularity small also stimulates a priority growth of silicon grain in a perpendicular direction with respect to the surface of the tin oxide, which produces collisions between crystal grains each growing from different surfaces of irregularities and having a different crystal orientation, but the defects are less generated since a difference in its orientation is small. Specifically, it is clear that the irregularities on the surface of the substrate are made as smaller as possible for reducing the defects in the crystalline silicon thin-film.
However, making no irregularities or making the irregularities on the surface small brings a result contrary to a technique for obtaining a highly efficient thin-film solar cell by enhancing the light confinement effect.
Japanese Laid-Open Patent Applications No. HEI 10-117006, No. HEI 10-294481, No. HEI 11-214728, No. HEI 11-266027 and No. 2000-58892 disclose a multi-junction type thin-film solar cell having a lower photoelectric conversion element that includes a photoelectric conversion layer made of a crystalline silicon layer and is formed on a back surface electrode on which irregularities are formed, this crystalline silicon layer having a (110) priority crystal orientation surface parallel to the substrate surface. However, these solar cells have an element structure of a sub-straight type in which light is incident from a photoelectric conversion element side.
Therefore, with respect to an element structure of a super-straight type in which light is incident from a transparent substrate side, no finding has been made as for a suitable rough structure achieving both of the reduction of defect density and light confinement effect in the crystalline silicon thin-film.
The present invention provides a thin-film solar cell comprising a set of a transparent conductive layer and a photoelectric conversion layer laminated in this order on a substrate, wherein the photoelectric conversion layer is made of a p-i-n junction, an i-layer constituting the p-i-n junction is made of a crystalline layer and the transparent conductive layer is provided with a plurality of holes at its surface of the side of the photoelectric conversion layer, each of said holes having irregularities formed on its surface.
The present invention also provides a method for manufacturing a thin-film solar cell characterized in that a surface of a substrate and/or a first transparent conductive layer is etched for forming a plurality of holes on the surface of the first transparent conductive layer upon manufacturing a thin-film solar cell of the above.
The present invention still provides a method for manufacturing a thin-film solar cell characterized in that a first transparent conductive layer is formed so as to have holes on its surface, whereby a plurality of holes are provided on the surface of the first transparent conductive layer upon manufacturing a thin-film solar cell of the above.
The present invention further provides a thin-film solar cell comprising two or more sets of a transparent conductive layer and a photoelectric conversion layer laminated in this order on a substrate, wherein a plurality of holes are provided on a surface at a side of a first photoelectric conversion layer of a first transparent conductive layer that is the closest layer to the substrate as well as on a surface at a side of a second photoelectric conversion layer of a second transparent conductive layer formed on the first transparent conductive layer, each of said holes having irregularities formed on its surface, the photoelectric conversion layer is made of a p-i-n junction, an i-layer constituting the p-i-n junction of the first photoelectric conversion layer is made of an amorphous or a crystalline layer and the i-layer of each of the other photoelectric conversion layers is made of a crystalline layer.
The present invention still another provides a method for manufacturing a thin-film solar cell characterized in that a surface of a substrate and/or a first transparent conductive layer and/or a second transparent conductive layer is etched for forming a plurality of holes on the surface of the first transparent conductive layer and on the surface of a second transparent conductive layer upon manufacturing a thin-film solar cell of the above.
The present invention still further provides a method for manufacturing a thin-film solar cell characterized in that a first transparent conductive layer and/or a second transparent conductive layer is formed so as to have holes on its surface, whereby a plurality of holes are provided on the surface of the first transparent conductive layer and on the surface of a second transparent conductive layer upon manufacturing a thin-film solar cell of the above.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.